The clear waters surrounding tropical coral reefs typically are oligotrophic, yet these reefs are highly productive and support dense populations of marine organisms. This paradox is resolved in part because many coral reef invertebrates accommodate unicellular autotrophs (‘‘zooxanthellae,’’ Symbiodinium spp.; Prochlorales, Prochloron sp.; cyanobacteria) within their tissues. These photoautotrophic symbioses entail an exchange of nutrients between the endosymbionts and the animal hosts. Organic… CONTINUE READING